A conductive hearing loss is a failure of the sound-conducting apparatus — the outer or middle ear — while the cochlea itself works normally. Its audiometric signature is unmistakable once you know what to look for: an air–bone gap.

The defining feature: the air–bone gap

Because the cochlea is healthy, bone conduction — which drives it directly — is normal. Air conduction, which must pass through the damaged conductive apparatus, is poorer. The vertical distance between the two curves is the air–bone gap, and a gap of roughly 10 dB or more is the hallmark of a conductive component.

Normal hearing

Otitis media with effusion

O Right ear, air
X Left ear, air
< Right ear, bone
> Left ear, bone

Signature: Flat, mild-to-moderate conductive loss with a uniform air–bone gap; normal bone conduction.

Figure 1. Conductive loss versus normal hearing. In the conductive ear, bone conduction stays near normal while air conduction is depressed — the gap between them is the air–bone gap.

Common causes

The conductive apparatus can fail at many points. In the outer ear, wax impaction or a canal that is blocked or collapsed. At the eardrum, perforation or scarring. In the middle ear, fluid (otitis media with effusion), a fixed stapes (otosclerosis), or a discontinuous ossicular chain. Each produces an air–bone gap, but the shape of the gap and the rest of the picture differ.

Trainee Configuration helps narrow the cause. Otitis media with effusion tends to give a fairly flat gap across the frequencies. Otosclerosis classically gives a gap that is largest in the low frequencies early on. A small ossicular problem may give a gap concentrated in the mid or low frequencies. The history — pain, discharge, the time course, noise exposure — does much of the remaining work.

Otosclerosis and the Carhart notch

Otosclerosis deserves special mention because of a characteristic twist. As otosclerotic bone fixes the stapes footplate, the audiogram shows the expected low-frequency conductive loss — but the bone-conduction curve also dips, typically by around 10–15 dB, with its lowest point at 2000 Hz. This dip is the Carhart notch.¹⁴

O Right ear, air
X Left ear, air
< Right ear, bone
> Left ear, bone

Signature: Low-frequency conductive loss with an air–bone gap; bone conduction shows the Carhart notch dipping at 2 kHz.

Figure 2. Otosclerosis with a Carhart notch. The conductive air–bone gap is present, and the bone-conduction curve dips at 2 kHz — the Carhart notch.

Clinician The Carhart notch is an artefact, not true cochlear loss. Bone-conduction testing partly relies on the normal resonance of the ossicular chain; fixing the stapes disturbs that resonance, depressing the measured bone-conduction threshold around 2 kHz. The practical consequence is important: the apparent sensorineural component of an otosclerotic audiogram is partly spurious, and the bone curve characteristically improves after successful stapes surgery as the notch resolves. Mistaking the notch for a real mixed loss can wrongly discourage surgery.

What the special tests add

A pure conductive loss produces no recruitment, so the SISI is negative, and it shows no abnormal adaptation, so tone decay is minimal. These findings are reassuring rather than diagnostic — they confirm the cochlea and nerve are behaving normally, which is consistent with the conductive picture the audiogram already shows. The real diagnostic work is the air–bone gap, the configuration, and the clinical history together.